The present invention pertains to prosthetic mechanical heart valves and in particular, to a bi-leaflet mechanical valve with an improved pivoting mechanism.
During each cardiac cycle, the natural heart valves selectively open to allow blood to flow through them and then close to block blood flow. During systole, the mitral and tricuspid valves close to prevent reverse blood flow from the ventricles to the atria. At the same time, the aortic and pulmonary valves open to allow blood flow into the aorta and pulmonary arteries. Conversely, during diastole, the aortic and pulmonary valves close to prevent reverse blood flow from the aorta and pulmonary arteries into the ventricles, and the mitral and tricuspid valves open to allow blood flow into the ventricles. The cardiac valves open and close passively in response to blood pressure changes operating against the valve leaflet structure. Their valve leaflets close when forward pressure gradient reverses and urges blood flow backward and open when forward pressure gradient urges blood flow forward.
In certain individuals, the performance of a natural heart valve is compromised due to a birth defect or becomes compromised due to various disease processes. Surgical repair or replacement of the natural heart valve is considered when the natural heart valve is impaired to an extent such that normal cardiac function cannot be maintained. The natural heart valve can be replaced by homograft valves obtained from the same species (e.g., human donor heart valves), heterograft valves acquired from different species, and prosthetic mechanical heart valves.
The present invention is directed to improvements in prosthetic mechanical heart valves. Modern implantable mechanical heart valves are typically formed of a relatively rigid, generally annular valve body defining a blood flow orifice and an annular valve seat and one or more occluders that are movable between a closed, seated position in the annular valve seat and an open position at an angle to the valve body axis. These components of mechanical heart valves are made of blood compatible, non-thrombogenic materials, i.e., pyrolytic carbon and titanium. A bio-compatible, fabric sewing ring is typically provided around the exterior of the valve body to provide an attachment site for suturing the valve prosthesis into a prepared valve annulus. The occluder(s) is retained and a prescribed range of motion is defined by a cooperating hinge mechanism or other restraining mechanism. Such prosthetic heart valves function essentially as check valves in which the occluder(s) responds to changes in the relative blood pressure in the forward and reverse directions as described above and move between their open and closed positions.
A wide variety of mechanical heart valve designs have been proposed and/or utilized in the past. For example, an early clinically used mechanical heart valve employed a spherical ball moving into and out of engagement with an annular seat within a cage in response to the normal pumping action of the heart. Other clinically used heart valve prostheses have employed occluders in the form of a circular disc that pivots open and closed in response to blood pressure changes while being restrained by cooperative structure of the valve body.
A further clinically used bi-leaflet heart valve prosthesis employs a pair of semi-circular or semi-elliptical plates or leaflets that hinge open and closed together. Such bi-leaflet heart valves are typically entirely formed with a pyrolytic carbon or with pyrolytic carbon coating on all exterior surfaces of the valve body and leaflets. A typical method of coating pyrolytic carbon onto a valve substrate is disclosed in U.S. Pat. No. 3,526,005. The pyrolytic carbon coating provides wear resistant surface, and provides insurance against thrombus formation on such surfaces.
Bi-leaflet heart valves generally utilize pivot or hinge mechanisms to guide and control the motion of the leaflets between the seated, closed position and the open position. In such design configurations, two mirror image leaflets are typically disposed in opposed or mirror image relation to one another. Upon closure, each valve leaflet occludes or covers half of the annular valve orifice or valve annulus. Generally, each leaflet is designed with roughly semicircular shape and has a rounded exterior margin and peripheral edge which engages an inner seat surface of the valve body to provide a peripheral seal, and an inner, diametrically extending edge and adjacent margins adapted to abut against the counterpart edges and margins on the other leaflet. Each leaflet can rotate about an axis defined by a pair of opposed hinge pivot points in opposed hinge recesses that are offset from the central axis of the valve annulus. The leaflets are typically flat, but curved or elliptical leaflets have been proposed.
Such mechanical heart valves are typically designed in somewhat differing profile configurations for replacement of differing impaired natural heart valves. However, the basic in vivo operating principle is similar regardless of configuration. Using an aortic valve as an example, when blood pressure rises in response to left ventricle contraction or systole in each cardiac cycle, the leaflets of such a valve pivot from a closed position to an open position to permit blood flow past the leaflets. When the left ventricle contraction is complete, blood tends to flow in the opposite direction in diastole in response to the back pressure. The back pressure causes the aortic valve leaflets to close in order to maintain arterial pressure in the arterial system.
The most widely accepted type of bi-leaflet heart valve presently used mounts its leaflets for pivoting movement by means of a pair of rounded ears extending radially outwardly from opposed edges of the leaflets to fit within rounded hinge recesses in opposed flat surfaces of the valve body side wall. Such bi-leaflet valves are exemplified by the mitral valve depicted in U.S. Pat. No. 4,276,658 and the aortic heart valve depicted in U.S. Pat. No. 5,178,632, both incorporated herein by reference.
The leaflet ears are received within curved hinge recesses extending radially into opposed flat surfaces of thickened wall sections inside the annulus of the generally cylindrical or annular valve body. Each hinge recess is designed in at least one respect to match the shape of the leaflet ear and is bounded by sets of leaflet stop surfaces angled to define the extreme open and closed leaflet positions. In other words, where the ear is formed as a portion of a circle having a given radius, the counterpart hinge recess is formed as a semicircle having a slightly greater radius. An inverse arrangement of the ear and recess hinge mechanism is depicted in U.S. Pat. No. 5,354,330, incorporated herein by reference, whereby the leaflet ear is replaced by a leaflet recess, and the hinge recess is replaced by a complementary shaped hinge boss.
To achieve the pivoting mechanism, the mating surfaces of the ears and recesses are precisely machined so as to provide a small but definite working clearance for the ears to pivot about the necked down pivot surface and be retained within the hinge recesses. During valve assembly, the annular valve body is deformed or distended so that the leaflet ears may be inserted into the respective hinge recesses. Each manufactured heart valve is then lab tested xe2x80x9cdryxe2x80x9d to ensure that the leaflets are held tightly enough to be secure against falling out of their hinge recesses, but are not so tightly engaged so as to create a binding or restricted valve action.
The range of leaflet motion is typically controlled by pins or ramps or opposed side stops of the hinge recesses or by hinge bosses in the valve body. In one format described in the above-incorporated ""632 patent, the hinge recess is generally spherical and bounded by open and closed stop surfaces of a stop member projecting into the recess. In the other formats depicted in the above-incorporated, ""658 and ""046 patents, each hinge recess has an elongated xe2x80x9cbow-tiexe2x80x9d or xe2x80x9cbutterflyxe2x80x9d appearance created by the inward angulation of opposed side edges extending from inflow and outflow end edges and meeting at opposite disposed, necked down, pivot points or surfaces intermediate the end edges.
A great deal of effort has been devoted to controlling the range of movement and the acceleration of the leaflets between the open and closed positions to both control noise and decrease wear or the possibility of leaflet fracture. In the past, bi-leaflet valves were known to be noisy, in the sense that patients could frequently hear the seating of the valve leaflet peripheral edges against the valve seats when they closed. It is desirable for patient comfort to provide a bi-leaflet design that minimizes the distraction of leaflet seating noise.
It is also known that blood cells are extremely fragile and delicate and can be damaged and/or destroyed when trapped in the valve seat during closure of the valve leaflet or in the wiping area of the valve leaflet ears and hinge recesses or between the leaflet ears and the open and closed stop surfaces. The wiping areas of the hinge recesses have the highest potential of thrombus formation and emboli entrapment which can accumulate therein, impair the movement of the valve leaflets, and result in valve failure requiring surgical intervention. The close tolerances and resulting narrow gaps between the leaflet ears and the hinge recess surfaces contribute to this problem. During the open leaflet phase, blood barely flows through the recess under very low forward pressure gradient. During the closed leaflet phase, blood flow in the hinge recess region is, of course, stopped. Any existing thrombus and/or entrapped emboli in the hinge recess can restrict leaflet motion, which in turn can further enhance thrombus formation and trapping more emboli. To this time, no design has been successful in eradicating this problem. Consequently, patients receiving current bi-leaflet mechanical heart valves are prescribed continuous blood anticoagulation therapy to prevent thrombus formation and thromboemboli.
In this regard, in prior art bi-leaflet prosthetic heart valves described above employing the generally concave hinge recesses formed in planar surfaces to receive generally convex leaflet ears, the side walls that stop movement of the leaflet ears and define the leaflet open and closed positions are designed to be orthogonally cut into the planar surfaces, resulting in relatively abrupt or xe2x80x9csharpxe2x80x9d edges with the planar surface. This configuration allows relatively large areas of contact between the sides of the valve ears and the recess side walls in the open and closed leaflet positions. The hinge recess end boundaries or edges extending between the ends of the side walls in the inflow and outflow directions are also designed with relatively sharp transitions at the planar surfaces. Relatively sharp corners are created at the junctions of the ends of the side walls and the ends of the inflow and outflow recess end edges. These sharp edges and corners create unnecessary blood flow stagnation regions and blood flow separations. The sharp edges and corners are also high mechanical stress concentration sites and may also be potential structural failure initiation sites.
In spite of significant advances which have taken place in the construction of mechanical heart valves, there is still room for significant improvements therein to minimize flow stagnation regions inside the valve annulus, especially in the hinge recesses, which may lead to thrombus formation and cause the failure of a prosthetic valve, to reduce impact force upon valve closure, and to eliminate flow turbulence in or near valve annulus.
This invention is therefore directed to improving the current hinge mechanisms employed in bi-leaflet prosthetic mechanical heart valves to increase blood washing of the hinge recess, improve valve hemodynamic performance, and to reduce leaflet impact force, valve failure potential, and the need for long term anticoagulant therapy.
Therefore, it is a primary object of the present invention to provide an improved mechanical heart valve prosthesis that provides improved performance while responding hemodynamically to the normal pumping action of the heart.
It is another object of the present invention to provide an improved heart valve prosthesis that reduces exposure of blood and its constituents to substantial mechanical forces and stresses, thereby reducing the likelihood of damage to constituents and cells found within human blood.
It is an additional object of the present invention to eliminate sharp boundaries or edges and corners where the end edges meet the side edges to avoid blood cell damage and to minimize potential mechanical failure initiation sites in the hinge recess.
It is a further object of the present invention to provide an improved heart valve prosthesis with mechanical design features and characteristics that reduce the occurrence of a concentration of mechanical stresses at given locations in the device.
It is yet a further object of the present invention to provide an improved heart valve prosthesis that enhances the uniformity of blood flow through the device so as to substantially reduce the creation of zones of stasis or stagnation within the device and its environs.
The designs of all mechanical, bi-leaflet prosthetic heart valves currently in clinical use ignore the differences in fluid dynamic forces acting on the leaflets between the opening phase and the closing phase. The contacting or bearing surfaces of the leaflet ear and hinge recess are typically symmetric and are simply designed to operate the same in both phases. The dynamic force applied to the leaflets in the valve closing phase is one to two orders of magnitude higher than that in the valve opening phase. We have realized that the hinge recess and matching leaflet ear should be designed so that the bearing surfaces in the inflow open phase and the outflow closing phase have differing configurations to account for and utilize the dynamic force difference. In accordance with the present invention, this dynamic force difference is taken into account, resulting in a hinge recess design having distinct features of the inflow and outflow portions thereof. These features include distinct inflow and outflow recess profiles and surface areas, differing side wall lengths, and differing inflow and outflow recess end edge heights.
In particular, in a first aspect of the present invention applicable to each hinge mechanism formed in opposed planar surface regions of the valve body interior wall, a relatively shallow inflow or entrance ramp is achieved in the hinge recess. A convexly curved inflow transition surface extends from the planar surface at the arcuate inflow hinge recess end boundary or edge into the generally concave inflow recess bearing surface. A relatively more open outflow or exit ramp is also employed through use of a further convexly curved outflow transition surface at the junction of the outflow recess bearing surface with the arcuate outflow hinge recess end boundary or edge. Moreover, a relatively recessed outflow flat in the relatively planar surface that the hinge recess is formed in extends in the outflow direction away from the arcuate outflow hinge recess end edge. The resulting recess bearing surface profile and the profile of the leaflet ear bearing edge contribute to favorable operation of the improved hinge configuration improving the blood flow dynamics, reducing areas of stasis, and reducing eddies in the blood flow pattern through the valve annular orifice.
The valve leaflet hinge mechanisms are preferably shaped to account for the differing separation distances between the planar surfaces at the inflow hinge recess end edge and the outflow flats at the outflow hinge recess end edges. To this end, the leaflet peripheral edge further comprises first and second relatively straight outflow shoulders extending between the opposite ends of the major arcuate peripheral edge of the leaflet and the ear bearing edge of the first and second leaflet ears, respectively, whereby the first and second outflow shoulders are separated apart by a distance somewhat less than the distance between the opposed outflow flats. Similarly, first and second relatively straight inflow shoulders extend between the opposite ends of the straight edge of the leaflet and the ear bearing edge of the first and second leaflet ears, respectively, whereby the first and second inflow shoulders are separated apart by a second distance less than the first distance and somewhat less than the distance between the opposed planar surfaces.
In addition, in a further aspect of the present invention, the relatively sharp or acute closed, and, optionally, open stop side edges of the hinge recess are replaced by convex stop edges at the junction of each side wall with the planar surface. The convex stop edge shape results in a contact band portion of each recess side wall projecting into each recess inflow and outflow portion to minimize the contact area with like contact band portions of the leaflet ear inflow and outflow sides. This reduces the total area of contact between each side wall and leaflet ear side, thereby reducing damage to blood cells in the contact region.
In a still further aspect of the present invention, the valve body is shaped to be generally annular with an annular interior side wall generally extending between an inflow rim and an outflow rim thereof. The annular valve body defines an annular blood flow orifice having a central blood flow axis centrally located with respect to the annular interior surface. A generally convex outflow rim transition surface extends in a band between the outflow rim and the annular interior side wall having a first radius of curvature. A generally convex inflow rim transition surface extends in a band between the inflow rim and the annular interior side wall having a second radius of curvature greater than the first radius of curvature. The inflow rim transition surface directs blood flow away from the annular interior surface and centrally through the annular orifice when the occluder means is in the open position.
In yet another aspect of the present invention, the parallel disposed and inwardly facing, opposed planar surfaces formed along the annular interior surface of the annular valve body are each bounded by planar surface side edges and planar surface inflow and outflow edges. The oppositely disposed of hinge recesses are each formed in one of the opposed planar surfaces for cooperatively engaging leaflet ears and for guiding movement of the leaflet ears between the leaflet open and closed positions. A planar inflow chamfer extends from the inflow rim to the planar surface inflow edge to deflect any blood components susceptible of damage by operation of the leaflet ears in the hinge recesses centrally through the annular orifice during blood flow therethrough.
In the preferred embodiments, the annular valve body includes a pair of such valve leaflets and respective pairs of hinge mechanisms for pivoting between open and seated, closed positions, thereby allowing a unidirectional flow of blood through the passageway in the open position during the cardiac cycle. The four hinge recesses are arranged in a mirror image relationship of oppositely disposed hinge recess pairs with respect to a pair of arcuate seats in respective halves of the valve body. The spacing apart of the oppositely disposed pairs of the hinge recesses provides for an optimized ratio of the central flow orifice between the two open leaflets in comparison to the two side flow orifices.
Each of the hinge recesses is butterfly or bow-tie shaped in outline, the outline defined by the arcuate inflow and outflow hinge recess end boundaries or edges that are separated by inflow and outflow pairs of opposed open and closed, stop side walls. The recess inflow and outflow side wall pairs extending toward one another are angled inward to centrally disposed, inflow and outflow pivots about which the valve leaflet ear (and the leaflet as a whole) pivots during the opening and closing phases.
The opposed pairs of elongated, butterfly or bow-tie shaped hinge recesses are generally angled, end-to-end, at common, and complementary, reference angles of inclination with respect to the center axis of the valve annulus. Each such reference angle of inclination is intermediate a maximum open angle of inclination of the open side wall and a maximum closed or seated angle of inclination of the closed side wall.
The inflow portion of the hinge recess in the planar surface extends from the inflow end edge and toward the pivot points and has a wide inflow mouth defined by the side edges thereof that diverge away from one another with distance from the pivot points and an inflow recess bearing surface. The inflow recess bearing surface is shaped from the inflow end edge to define an entrance ramp in the inflow portion to admit blood flow into the hinge recess in the inflow direction. The entrance ramp slope allows good blood flow washing function in this inflow portion of the hinge recess while the inflow recess bearing surface is wiped as the valve leaflet ear sweeps over it in pivoting between the open and closed positions.
The design of the hinge recess also provides controlled pivoting and translation guidance to the movement of a valve leaflet into the closed and seated position. This allows a leaflet to change its rotating axis near its closed position and reduce its tangential velocity at the leaflet major radius. The reduction in velocity reduces impact force of contact of the leaflet""s peripheral seat edge with the valve seat in the valve body. This reduced impact force prolongs the heart valve""s fatigue life, and reduces the propensity of cavitation and valve closing noise. The large surface area of contact of the leaflet ear bearing edge with the inflow portion of the hinge recess reduces concentrated valve closing stress on the hinge recess.
The outflow portion of the hinge recess extends between the centrally disposed pivot points and the outflow end of the recess, and the outflow bearing surface in the outflow portion is asymmetric in contour with respect to the inflow bearing surface. The side edges of the recess also diverge apart from the centrally disposed pivot points to the point where they meet the arcuate outflow end edge.
The decreased height of the junction of the outflow hinge recess end edge with the outflow flat allows blood funneled into the hinge recess in the inflow direction to more readily flow out again into the outflow flat in the outflow direction. The convexly shaped leaflet ears projecting into the hinge recesses allow the leaflet to pivot between the open and closed positions and sweep the outer edges of the leaflet ears over the recess bearing surface and flush blood components from the hinge recesses and into the outflow flats and from the outflow flats through the annular orifice of the heart valve. This open exit takes advantage of a much smaller dynamic drag force acting on the opposed flat leaflet surfaces during the valve opening phase than exist during the valve closing phase. Any blood components located within the hinge recess can be easily washed out by the blood flow in both the inflow and outflow directions while the leaflet ears sweep across the inflow and outflow portions of the hinge recess base in both the opening and closing directions.